1. Field of the Invention
The present invention relates to an optical pickup unit and an optical disk apparatus that are capable of reproducing data recorded on media such as various types of optical disks and of recording data on media such as various types of optical disks.
2. Description of the Related Art
FIGS. 16 to 23 relate to a conventional optical pickup unit and a conventional optical disk apparatus.
With the use of an optical disk apparatus equipped with an optical pickup unit, optical signals are read out from an optical disk or written onto the optical disk. Within the optical disk apparatus equipped with the optical pickup unit, in order for a focusing spot to be correctly applied on a predetermined track of the optical disk, a tracking control is performed by detecting a tracking error indicative of deviation of the focusing spot relative to a desired track and adjusting a position of an objective lens based on such tracking error. The tracking by the optical pickup unit means the operation of following the fluctuation in the radial direction of the optical disk so that the focusing spot is always located on a target track. A tracking error detection method can be, e.g., a differential push-pull method (DPP method) using three beams.
Media such as optical disks can be, e.g., a DVD (Digital Versatile Disk (registered trademark). Media means objects for recording and conveying information on and through or the like. DVD can be, e.g., DVD-R (Recordable), DVD-RW (Re-writable), DVD-RAN (Random Access Memory), etc. A track pitch of DVD-R or CVD-RW is about 0.74 μm (micron), while that of DVD-RAM (versions 2.0, 2.1) is about 0.615 μm (micron). As seen, the tracking pitch differs between DVD-R or DVD-RW and DVD-RAM.
In the optical pickup unit and the optical disk apparatus with the tracking detection method being the DPP method, in the case of using an optical disk with a predetermined track pitch, no “phase difference” occurs between a leading sub-push-pull (leading sub-PP) signal and a trailing sub-push-pull (trailing sub-PP) signal, as shown in FIG. 16. If a “phase difference” occurs between the leading sub-push-pull (leading sub-PP) signal and the trailing sub-push-pull (trailing sub-PP) signal, such a “phase difference” is called, for example, a tracking error phase difference (TE phase difference).
For example, if the amount of the TE phase difference and the amount of eccentricity of the optical disk are both small, a tracking error signal (TE signal) of such a waveform as shown in FIG. 17 is obtained. When the TE signal of the waveform shown in FIG. 17 is observed, the performance of recording or reproducing onto or from the optical disk is not adversely affected and an almost stabilized tracking servo function works, while the tracking error detection is being performed at the optical pickup unit.
In the optical pickup unit and the optical disk apparatus with the tracking detection method being the DPP method, in the case of using an optical disk with another than the predetermined track pitch, a “phase difference” occurs between the leading sub-push-pull (leading sub-PP) signal and the trailing sub-push-pull (trailing sub-PP) signal, as shown in FIG. 18. Namely, the TE phase difference occurs between the leading sub-push-pull (leading sub-PP) signal and the trailing sub-push-pull (trailing sub-PP) signal.
When the TE phase difference occurs, the amplitude level and the signal quality of the TE signal deteriorate and the performance can be affected of recording or reproducing onto or from the optical disk. When the amount of the eccentricity of the optical disk is big, this effect becomes conspicuous, the operation of the tracking servo becomes destabilized, and the basic operation of the optical pickup unit can be adversely affected.
For example, when the amount of the TE phase difference is big and the amount of eccentricity of the optical disk is considerable, the TE signal of such a waveform as shown in FIG. 19 is obtained. When the TE signal waveform has, for example, a significantly shrunk part as shown in FIG. 19, the TE signal level comes to be considerably reduced due to the eccentricity of the optical disk, etc., and in the worst case, the tracking servo could possibly fail. Therefore, the amount of the TE phase difference should preferably be suppressed to be as small as possible.
Recently, there has been an increasing demand for the optical pickup unit or the optical disk apparatus capable of recording and reproducing data onto or from plural kinds of the optical disks with different track pitches. To respond to such a demand, for example, the unit or apparatus was proposed that was capable of performing the tracking control by detecting the tracking error in a manner not dependent on the track pitch.
Proposed, for example, was a tracking error detecting apparatus for an optical head that is capable of suppressing the offset caused by translational motion of an objective lens or inclination of an information recording medium while maintaining the amplitude of the tracking error signal at a maximum level even if the track pitch of the information recording medium varies and moreover of which the optical system is simple.
Such a conventional apparatus is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. H09-81942 (pp. 1, 3, and FIGS. 1 to 7).
Into the tracking error detecting apparatus for the optical head described in the above-referenced patent application, the introduction is made of what resembles a diffraction grating 80 shown in FIG. 20. The diffraction grating 80 has phase shift regions 81 and 82 (FIG. 21) that cause a phase shift of n radian in part of laser light applied from a laser diode. The diffraction grating 80 (FIG. 20) is divided into two regions, the substantially rectangular first region 81 and the substantially rectangular second region 82 adjacent to the first region 81. A predetermined periodic structure is built within each of regions 81 and 82.
For example, the optical pickup and the optical information recording apparatus or reproducing apparatus utilizing the optical pickup were proposed which pickup, while retaining advantages of a new tracking error signal detection method (in-line DPP method), alleviated the problem of field characteristic deterioration of the tracking error signal which problem is the greatest with the in-line DPP method, was less dependent on differences in track pitch between disks and on displacement of the objective lens in a tracking direction, and was capable of outputting a practical tracking error signal with the tracking offset sufficiently reduced for any of plural kinds of optical disks of different pitches.
Such a conventional apparatus is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2004-145915 (pp. 5-6, FIGS. 1, 7 to 22).
The field characteristic is characterized as the degree of deterioration of the tracking error signal due to a displacement in the tracking direction of the objective lens relative to the disk.
In the optical pickup and the optical information recording or reproducing apparatus utilizing the optical pickup described in the above-identified patent application, the introduction is made of what resembles a diffraction grating 90 shown in FIG. 22. The diffraction grating 90 has phase shift regions 91 and 93 that cause a phase shift of n radian in part of the laser light applied from the laser diode (FIG. 23). The diffraction grating 90 (FIG. 22) is divided into three regions, a substantially rectangular first region 91, a short-width second region 92 adjacent to the first region 91 and a substantially rectangular third region 93 adjacent to the second region 92. A predetermined periodic structure is built within each of regions 91, 92, and 93.
However, the optical pickup unit, and optical disk apparatus, equipped with the conventional diffraction grating 80 (FIG. 20) and the optical pickup unit, and optical disk apparatus, equipped with the conventional diffraction grating 90 (FIG. 22) were not considered to demonstrate entirely satisfactory performance.
Among the tracking characteristics of the optical pickup unit, the field characteristic is considered to be an important characteristic. However, the field characteristic and the tracking error phase difference characteristic are, in terms of performance, in a relationship of tradeoff. Here, the tradeoff means the relationship in which a plurality of conditions can not be met at the same time.
In the optical pickup unit equipped with the conventional a 3-division-type diffraction grating 90 (FIG. 22), the field characteristic or the tracking error phase difference characteristic can be changed only by the width 90w of a center division part 90m of the three-division-type diffraction grating 90. For this reason, both of the satisfactory field characteristic and tracking error phase difference characteristic was not necessarily realized, both of which are necessary for the drive side of the optical disk apparatus.
As shown in FIG. 22, the three-division-type diffraction grating 90 does not have a borderline for equally dividing the diffraction grating 90 in plan view. The borderline for equally dividing the diffraction grating is used, for example, as a positioning centerline for fixing the diffraction grating, while accurately adjusting its positioning, in a housing of the optical pickup unit. With no positioning centerline provided in the diffraction grating 90, it was difficult to accurately mount the diffraction grating 90 in the housing of optical pickup unit, using an optical axis adjusting camera (not shown). If the diffraction grating 90 is not accurately mounted in the housing of the optical pickup unit, the focusing spot may not accurately be applied and formed on the optical disk.
Recently, there has been a demand from the market for a single, high-grade optical pickup unit that is easy to perform the tracking control, and the optical disk apparatus equipped with the single, high-grade optical pickup unit that is easy to perform the tracking control in order that they can handle without difficulty plural kinds of optical disks having different track pitches, such as DVD-R, DVD-RW, DVD-RAM, etc.
For example, there is a market demand for the optical pickup unit that has no deterioration of the amplitude of the tracking error signal due to the displacement of the objective lens, or for the optical pickup unit that prevents an offset from remaining in the tracking error signal at the time of recording or reproducing data onto or from plural kinds of optical disks having different track pitches.
For example, there is a market demand for the optical pickup unit that can demonstrate optimum characteristics where the optical pickup unit is utilized. For example, the optical pickup unit used for optical disk apparatuses for notebook-type or laptop-type personal computers utilizes a small objective lens.
Personal computers (PCs) will now be described. A notebook-type or laptop-type PC, in view of the demand for ones of a lighter weight and a smaller thickness, is structured to house the optical disk apparatus equipped with a slim drive. The notebook-type PC or the laptop-type PC has its display and PC body structured as one unit, and by folding its display back toward the PC body, becomes of a thin size. The notebook-type PC is, for example, a general-use PC of a substantially A4 size or a smaller size as viewed in plan and is also called a book-type PC. The notebook-type PC or the laptop-type PC is characterized as a compact and easy-to-carry PC.
With the notebook-type PC and the laptop-type PC becoming smaller and thinner, the optical disk apparatus is becoming smaller and thinner. With the optical disk apparatus becoming smaller and thinner, the optical pickup unit is becoming smaller and thinner. With the optical pickup unit becoming smaller and thinner, the objective lens mounted in the optical pickup unit is becoming smaller and thinner. Since a small objective lens is considered to be disadvantageous in respect of the field characteristic, much importance is attached mainly to the field characteristic, in the case of the small objective lens. Accordingly, in designing of the optical pickup unit equipped with the small objective lens, much importance is attached mainly to the field characteristic.
For example, in the optical pickup unit used for the optical disk apparatus for the desktop-type PC, the field characteristic is considered to be important, but at the same time, more importance may be attached mainly to a sub-push-pull signal amplitude level characteristic (sub-PP signal amplitude level characteristic) and a tracking error phase difference characteristic (TE phase difference characteristic), than to the field characteristic. A desktop computer is a computer that can be used on desk, and is of the type that is not easy to carry.
Because the optical pickup unit used for the optical disk apparatus for the desktop-type PC has a large objective lens, the tendency exists that in designing, more importance is attached mainly to the sub-PP signal amplitude level characteristic and the TE phase difference characteristic, than to the field characteristic. As such, an optical pickup unit is in demand that can readily demonstrate optimum characteristics corresponding to the specifications, etc., of the optical disk apparatus on which the optical pickup unit is mounted.